A THz-time domain spectroscopy (THz-TDS) apparatus (THz-TDS is also called terahertz time domain spectroscopy) acquires the electric field intensity of a terahertz wave (in this description, terahertz waves refers to electromagnetic waves within at least a partial frequency band of a range from 0.03 to 30 THz) that reaches a detection unit by an ultrashort pulse. The electric field intensity is continuously recorded while the timing at which the ultrashort pulse reaches the detection unit is changed. Accordingly, the time waveform of the terahertz wave can be acquired. Also, when characteristic absorption of the object is measured, the object is irradiated with the terahertz wave, and the time waveform of the terahertz wave from the object is acquired. Then, the acquired time waveform is transformed by Fourier transform, so that the time waveform is converted into information of a frequency domain, and a frequency spectrum can be obtained.
If interfaces (an interface is a portion at which a refractive index changes) are present in the object, a time waveform that contains reflected waves from the interfaces in the object can be acquired. Thus, the inner structure of the object can be acquired.
At this time, an interference fringe may appear in the frequency spectrum because of the reflected waves from the interfaces. The interference fringe may become a noise when the frequency spectrum of the object is analyzed. To reduce the influence of such an interference fringe, a method is described in Japanese Patent Laid-Open No. 2004-003902 as follows. After the time waveform of the terahertz waves containing the reflected waves from the interfaces is acquired, a time waveform relating to a reflected wave from an undesirable interface is eliminated from the acquired time waveform.